Critical Public Variable Without Final Modifier

Description

If a field is non-final and public, it can be changed once the value is set by any function that has access to the class which contains the field. This could lead to a vulnerability if other parts of the program make assumptions about the contents of that field.

Background

Mobile code, such as a Java Applet, is code that is transmitted across a network and executed on a remote machine. Because mobile code developers have little if any control of the environment in which their code will execute, special security concerns become relevant. One of the biggest environmental threats results from the risk that the mobile code will run side-by-side with other, potentially malicious, mobile code. Because all of the popular web browsers execute code from multiple sources together in the same JVM, many of the security guidelines for mobile code are focused on preventing manipulation of your objects' state and behavior by adversaries who have access to the same virtual machine where your program is running.

Final provides security by only allowing non-mutable objects to be changed after being set. However, only objects which are not extended can be made final.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

Suppose this WidgetData class is used for an e-commerce web site. The programmer attempts to prevent price-tampering attacks by setting the price of the widget using the constructor.

public final class WidgetData extends Applet {
  public float price;
  ...
  public WidgetData(...) {
    this.price = LookupPrice("MyWidgetType");
  }
}

The price field is not final. Even though the value is set by the constructor, it could be modified by anybody that has access to an instance of WidgetData.

Example Two

Assume the following code is intended to provide the location of a configuration file that controls execution of the application.

public string configPath = "/etc/application/config.dat";

public String configPath = new String("/etc/application/config.dat");

While this field is readable from any function, and thus might allow an information leak of a pathname, a more serious problem is that it can be changed by any function.

See Also